Arterial and venous blood metrics

ABSTRACT

A medical device including a probe configured to be orally inserted into a lumen extending into the thorax of the subject, a plurality of electrodes, and a control circuit. The probe includes a first electrode. The plurality of electrodes includes at least one second electrode and at least one third electrode configured to be disposed externally on the thorax of the subject on a first side of a sternum of the subject and a second side of the sternum of the subject, respectively, the second side opposite the first side. The control circuit is electrically coupled to the first electrode and the at least one second and third electrodes and configured to measure an impedance between the first electrode and each of the at least one second and third electrodes and determine a ratio of arterial blood volume relative to venous blood volume based upon the measured impedance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/957,091 entitled “Arterial and Venous Blood Metrics”, filed on Aug.1, 2013, which claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 61/679,275 entitled “Arterial andVenous Blood Metrics”, filed Aug. 3, 2012, and to U.S. ProvisionalApplication Ser. No. 61/684,435 entitled “Arterial and Venous BloodMetrics”, filed Aug. 17, 2012, all of which are incorporated herein byreference in their entireties.

BACKGROUND

Embodiments of the present invention are generally directed to systemsand methods of determining various metrics relating to the ratio ofarterial and venous blood in the circulatory system of a subject, andmore particularly to systems and methods of determining various metricsrelating to the ratio of arterial and venous blood in the circulatorysystem of a subject as an indicator of the quality of Cardio PulmonaryResuscitation (CPR).

SUMMARY

In accordance with an aspect of the present invention, a medical deviceis provided. The medical device comprises a probe that includes a firstelectrode and which is configured to be orally inserted into a lumen ofa subject that extends into the thorax of the subject, a plurality ofelectrodes, and a control circuit. The plurality of electrodes includesat least one second electrode configured to be disposed externally onthe thorax of the subject on a first side of a sternum of the subjectand at least one third electrode configured to be disposed externally onthe thorax of the subject on a second side of the sternum of the subjectthat is opposite the first side. The control circuit is electricallycoupled to the first electrode, the at least one second electrode, andthe at least one third electrode. The control circuit is configured tomeasure an impedance between the first electrode and each of the atleast one second and third electrodes and determine a ratio of arterialblood volume relative to venous blood volume based upon the measuredimpedance.

In some embodiments, the control circuit is further configured tomeasure the impedance between the first electrode and each of the atleast one second and third electrodes over a period of time, and todetermine a rate of change of the ratio of the arterial blood volumerelative to the venous blood volume over the period of time. In variousembodiments, the control circuit is further configured to issue arecommendation to modify an intravascular fluid volume of the subjectbased at least in part on the determined rate of change of the ratio ofthe arterial blood volume relative to the venous blood volume over theperiod of time. In other embodiments, the control circuit is furtherconfigured to issue a recommendation to modify an intravascular fluidvolume of the subject based at least in part on the determined ratio ofarterial blood volume relative to venous blood volume.

In accordance with some embodiments, the control circuit is furtherconfigured to determine a ratio of the arterial blood volume relative toa capacity of the arterial blood system of the subject based upon themeasured impedance, to determine a ratio of the venous blood volumerelative to a capacity of the venous blood system of the subject basedupon the measured impedance, or both.

In various embodiments, the control circuit is further configured togenerate a metric indicative of a quality of cardio pulmonaryresuscitation being performed on the subject. In some embodiments, thecontrol circuit is further configured to issue a recommendation tomodify the cardio pulmonary resuscitation being performed on the subjectbased on the metric. The recommendation can include at least one ofvarying the rate of cardio pulmonary resuscitation compressions, varyinga depth of the cardio pulmonary resuscitation compressions, varying aposition at which the cardio pulmonary resuscitation compressions areapplied to the subject, varying a direction at which the cardiopulmonary resuscitation compressions are applied to the subject,changing a duration of time over which each compression is applied tothe subject, changing a duration of time over which compressive forcesare released, and recommending administration of a pharmacologicalcompound.

In accordance with some embodiments, the medical device includes a userinterface, and the control circuit is further configured to at least oneof audibly and visually present the recommendation on the userinterface. In various embodiments, the medical device includes acommunication interface by which it can communicate with anelectromechanical chest compression device, and the control circuit isfurther configured to communicate the recommendation to theelectromechanical chest compression device. In various embodiments, themedical device can communicate via the communication interface with aresuscitation device, such as a chest compression device, a CPR coachingand feedback device, or a defibrillator.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 illustrates the circulatory system of a human subject;

FIG. 2 illustrates a medical device that may be used to monitor thethoracic impedance of a subject and to determine various metricsrelating to the ratio of arterial and venous blood in the circulatorysystem of the subject in accordance with an embodiment of the presentinvention;

FIG. 3 illustrates a portion of the medical device of FIG. 2 thatincludes a probe and a plurality of external electrodes in accordancewith an embodiment of the present invention;

FIG. 4 is a plan view of the probe of FIG. 3;

FIG. 5 is a tomography map of the electrical impedance of varioustissues of a human body; and

FIG. 6 is vertical cross-sectional view of the conductance of variousinternal organs, tissues, muscles, and bones of a human body.

DETAILED DESCRIPTION

Embodiments of the invention are not limited to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. Embodiments of theinvention are capable of being practiced or of being carried out invarious ways. Also, the phraseology and terminology used herein is forthe purpose of description and should not be regarded as limiting. Theuse of “including,” “comprising,” or “having,” “containing,”“involving,” and variations thereof herein, is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

Cardiac arrest is a major cause of death worldwide. Variousresuscitation efforts aim to maintain the body's circulatory andrespiratory systems during cardiac arrest in an attempt to save the lifeof the subject. Such resuscitation efforts may include CPR (i.e., chestcompressions with or without artificial respiration). A number ofdifferent devices are commercially available to improve theeffectiveness of CPR, including manually operated devices, such as thePocketCPR®. CPR coaching and feedback device, and electromechanicalchest compression devices, such as the AutoPulse® Non-invasive CardiacSupport Pump, each available from ZOLL Medical Corporation, ofChelmsford, Mass.

Applicants have determined that various metrics relating to the ratio ofarterial and venous blood in the circulatory system of a subject may beused an indicator of the quality of Cardio Pulmonary Resuscitation (CPR)being performed on the subject. Such metrics may include a ratio of thearterial blood volume relative to the venous blood volume at aparticular instant of time, changes in the ratio of arterial bloodvolume relative to the venous blood volume over a period of time, therate of filling of the arterial and venous vessels over time, changes inthe rate of filling of the arterial and venous vessels over a period oftime, a ratio of the capacity of arterial blood volume relative to thecapacity of venous blood volume at a particular instant of time, changesin the capacity of arterial blood volume relative to the capacity ofvenous blood volume over a period of time, the directionality (i.e.,retrograde or antegrade) of blood flow, and changes in thedirectionality of blood flow over a period of time.

Applicants have determined that these metrics can be indicative of thequality of CPR being performed on a subject, and further, that suchmetrics may be used to modify the CPR being performed on the subject toenhance the quality of the delivered CPR. Such modifications may includeadjusting the location at which compressions are applied to the subject,adjusting a direction in which compressions are applied to the subject,adjusting the frequency, depth, or duration of the CPR compressions,adjusting the amount of time over which the compressive forces arereleased, etc.

FIG. 1 illustrates the circulatory system of human subject. Asillustrated in FIG. 1, the circulatory system includes both a venousportion that includes the vena cava 10 and its associated veins and anarterial portion that includes the aorta 12 and its associated arteries.As shown in FIG. 1, both the trachea 14 and the esophagus 16 of thesubject descend centrally into the subject's thorax, with the greatvessels of the venous portion (e.g., the vena cava) being disposedsubstantially to the right of the subject's trachea or esophagus andwith the great vessels of the arterial portion (e.g., the aorta) beingdisposed substantially to the left of the subject's trachea oresophagus.

Various metrics relating to the ratio of arterial and venous blood inthe circulatory system of a subject can be indicative of the quality ofCPR that is being performed on the subject. In general, during effectiveCPR, the relative volume of blood in the arterial portion of thesubject's circulatory system should be greater than that in the venousportion. By monitoring various metrics relating to the ratio of thevolume of arterial blood relative to the volume of venous blood in thecirculatory system of the subject, the effectiveness of CPR can beincreased and the CPR may be tailored to adjust to the individualcharacteristics of the subject.

In accordance with an aspect of the present invention, a medical deviceis provided that is capable of monitoring various metrics relating tothe ratio of arterial and venous blood in the circulatory system of asubject. In accordance with one embodiment, the medical device measuresthe impedance of the great vessels (e.g., the aorta and the vena cava)to determine a ratio of the arterial blood volume relative to the venousblood volume at a particular instant of time, changes in the ratio ofarterial blood volume relative to the venous blood volume over a periodof time, the rate of filling of the arterial and venous vessels overtime, changes in the rate of filling of the arterial and venous vesselsover a period of time, a ratio of the capacity of arterial blood volumerelative to the capacity of venous blood volume at a particular instantof time, and changes in the capacity of arterial blood volume relativeto the capacity of venous blood volume over a period of time. In someembodiments, the medical device may be capable of also determining thedirectionality of blood flow and changes in the directionality of bloodflow over a period of time. Although aspects and embodiments of thepresent invention are primarily directed to the use of impedancemeasurements to determine the various metrics, those skilled in the artshould appreciate that other types of measurements may alternatively oradditionally be used, such as ultrasound.

As shown in FIG. 2, the medical device 200 includes a control circuit205 that is electrically coupled to a probe 240 that includes at leastone first electrode 241 and a plurality of additional electrodes 250including at least one second electrode 250 a, and at least one thirdelectrode 250 b. As described in more detail with respect to FIGS. 3 and4 below, the probe 240 is configured to be orally inserted into a lumenof the subject, such as the trachea or the esophagus of the subject, andthe plurality of additional electrodes 250 are configured to be disposedexternally on the thorax of the subject.

As shown in FIG. 2, the control circuit 205 may be implemented on acomputer system, such as a personal computer or workstation. It shouldbe appreciated that embodiments of the present invention are not limitedto executing on any particular type of computer system, as variousaspects of the present invention may be implemented in software,hardware or firmware, or any combination thereof, includingspecially-programmed hardware and/or software.

As depicted, the control circuit 205 includes a processor 210, a memory212, a bus 214, a communication interface 216, a user interface 220, anelectrode interface 230 and a storage system 218. The processor 210,which may include one or more microprocessors or other types ofcontrollers, can perform a series of instructions that manipulate data,including data provided by the electrodes 241, 250 a, and 250 b. Theprocessor 210 may be a well-known, commercially available processor suchas an Intel Pentium, Intel Atom, ARM Processor, Motorola PowerPC, SGIMIPS, Sun UltraSPARC, or Hewlett-Packard PA-RISC processor, or may beany other type of processor or controller as many other processors andcontrollers are available. The processor 210 may execute an operatingsystem which may be, among others, a Windows-based operating system (forexample, Windows NT, Windows 2000/ME, Windows XP, Windows 7, or WindowsVista) available from the Microsoft Corporation, a MAC OS System Xoperating system available from Apple Computer, one of many Linux-basedoperating system distributions (for example, the Enterprise Linuxoperating system available from Red Hat Inc.), a Solaris operatingsystem available from Sun Microsystems, or a UNIX operating systemsavailable from various sources. Many other operating systems may beused, and embodiments are not limited to any particular operatingsystem.

The processor and operating system together define a computing platformfor which application programs in high-level programming languages maybe written. These component applications may be executable, intermediate(for example, C# or JAVA bytecode) or interpreted code which communicateover a communication network (for example, the Internet) using acommunication protocol (for example, TCP/IP). Similarly, functions inaccord with aspects of the present invention may be implemented using anobject-oriented programming language, such as SmallTalk, JAVA, C++, Ada,or C# (C-Sharp). Other object-oriented programming languages may also beused. Alternatively, procedural, scripting, or logical programminglanguages may be used.

As shown, the processor 210 is connected to the other control circuitcomponents, including the memory 212, the storage system 218, and theinterfaces 216, 220, and 230, by the bus 214. The memory 212 may be usedfor storing programs and data during operation of the control circuit205. Thus, the memory 212 may be a relatively high performance,volatile, random access memory such as a dynamic random access memory(DRAM) or static memory (SRAM). However, the memory 212 may include anydevice for storing data, such as a disk drive or other non-volatilestorage device, such as flash memory or phase-change memory (PCM).Various embodiments in accord with the present invention can organizethe memory 212 into particularized and, in some cases, unique structuresto perform the aspects and functions disclosed herein.

The various components of the control circuit 205 are coupled to oneanother by an interconnection element such as the bus 214. The bus 214may include one or more physical busses (for example, busses betweencomponents that are integrated within a same machine), and may includeany communication coupling between control circuit components includingspecialized or standard computing bus technologies such as IDE, SCSI,PCI and InfiniBand. Thus, the bus 214 enables communications (forexample, data and instructions) to be exchanged between the variouscomponents of the control circuit 205.

The control circuit 205 also includes a number of interfaces including acommunication interface 216, a user interface 220, and an electrodeinterface 230. The control circuit 205 may communicate with anotherdevice 260, such as an electromechanical chest compression device, aBasic Life Support (BLS) device, or an Advanced Life Support (ALS)device, such as the AED Pro or AED Plus defibrillator, LifeVest® ProPackM. D., X Series defibrillators, M Series defibrillators, R Seriesdefibrillator BLS, or E Series defibrillator manufactured by the ZOLLMedical Corporation of Chelmsford Mass. via the communication interface216. As described more fully below, where the device 260 is capable ofproviding CPR to a subject, the control circuit 205 may instruct theother device 260 to vary the CPR protocol being delivered to the subjectbased on the various metrics. Alternatively, where the other device 260is an ALS device, the control circuit 205 may provide data and/or thevarious metrics to the other device for storage or display, or to enablethe other device to guide or provide feedback to a rescuer based on theprovided data or metrics.

According to a variety of examples, the communications interface 216 maysupport a variety of standards and protocols, examples of which includeUSB, TCP/IP, Ethernet, Wireless Ethernet, Bluetooth, ZigBee, M-Bus, IP,IPV6, UDP, DTN, HTTP, FTP, SNMP, CDMA, NMEA and GSM. To ensure datatransfer is secure, in some examples, the control circuit 205 cantransmit data via the communication interface 216 using a variety ofsecurity measures including, for example, TSL, SSL or VPN. In otherexamples, the communication interface 216 includes both a physicalinterface configured for wireless communication and a physical interfaceconfigured for wired communication.

The user interface 220 includes a combination of hardware and softwarecomponents that allow the control circuit 205 to communicate with anexternal entity, such as a user. These components are configured toreceive information from actions such as physical movement, verbalintonation or thought processes. In addition, the components of the userinterface 220 can provide information to external entities. Examples ofthe components that may be employed within the user interface 220include keyboards, mouse devices, trackballs, microphones, touchscreens, printing devices, display screens and speakers. For example,the user interface 220 may be used to visually or audibly present ametric to a user relating to the effectiveness of the CPR beingdelivered, or to provide instructions as to how the CPR should be variedto increase the effectiveness of the CPR.

The electrode interface 230 is configured to receive electrical signalsmonitored by the various electrodes 241, 250 a, 250 b and to providethose electrical signals to the processor 210. The electrode interface230 may include selection circuitry, such as a multiplexer, to selectfrom among the different pairings of electrodes and to provideelectrical signals to the processor. The electrode interface 230 mayalso include a plurality of differential amplifiers to receive, buffer,filter, and amplify the electrical signals provided by the differentpairings of electrodes, and one or more Analog-to-Digital (A/D)converters to convert the received analog signals to a digital signalthat is provided to the processor 210.

Storage system 218 may include a computer-readable andcomputer-writeable nonvolatile storage medium in which instructions arestored that define a program to be executed by the processor. Such aprogram executed by the processor may include instructions to calculatea ratio of arterial blood volume and venous blood volume at a particularinstant of time, or over a period of time. The storage system 218 alsomay include information that is recorded, on or in, the medium, and thisinformation may be processed by the processor 210. More specifically,the information may be stored in one or more data structuresspecifically configured to conserve storage space or increase dataexchange performance. The instructions may be persistently stored asencoded signals, and the instructions may cause the processor to performany of the functions described herein. A medium that can be used withvarious embodiments may include, for example, optical disk, magneticdisk or flash memory, among others. In operation, the processor 210 maycause data to be read from the nonvolatile recording medium into anothermemory, such as the memory 212, that allows for faster access to theinformation by the processor 210 than does the storage medium includedin the storage system 218. The processor 210 may manipulate the datawithin the memory 212, and then copy the data to the medium associatedwith the storage system 218 after processing is completed. A variety ofcomponents may manage data movement between the medium and the memory212, and the invention is not limited thereto.

FIG. 3 illustrates a portion of the medical device of FIG. 2 in whichthe probe 240 is inserted into a lumen of the subject and the pluralityof additional electrodes are disposed externally on the thorax of thesubject. As described with respect to FIG. 2, the probe includes atleast one first electrode, and the plurality of additional electrodesincludes at least one first electrode and at least one second electrode.In the embodiment illustrated in FIG. 3, the probe 240 includes aplurality electrodes 341 a and 341 b, 342 a and 342 b, and 343 a and 343b disposed at spaced apart positions along a length of the probe 240.The electrodes 341, 342, 343 may be any type of electrode suitable foruse inside the body of a subject, and may be mounted to an externalsurface of the probe 240, or integrated therein. It should beappreciated that rather than including pairs of electrodes (e.g., 341 a,341 b) disposed on opposing side surfaces of the probe 240, only asingle electrode (e.g., 341 a, 342 a, 343 a) may be provided at eachlevel along the length of the probe 240, and that in certainembodiments, only a single electrode (e.g., 342 a) may be provided.However, the use of multiple electrodes disposed in spaced apartpositions along the length of the probe permits impedance measurementsto be made in multiple axes.

As shown in FIG. 3, the plurality of additional electrodes includes atleast one second electrode 350 a disposed externally on the thorax ofthe subject on a first side of the sternum of the subject and at leastone third electrode 350 b disposed externally on the thorax of thesubject on a second side of the sternum of the subject that is oppositethe first side. The placement of the at least one second electrode 350 aand the at least one third electrode 350 b permits impedancemeasurements of the arterial circulatory system and the venouscirculatory system. The plurality of additional electrodes 350, 351, 352may be any type of electrode suitable for external use on the body ofthe subject, such as wet or dry self-adhesive medical electrodestypically used to measure electrical signals on the body of a subject.

As illustrated in FIG. 3, the plurality of additional electrodes mayfurther include additional electrodes disposed along the length andwidth of the thorax of the subject. For example, as shown in FIG. 3, theplurality of additional electrodes may include electrodes 350 a and 350c and 350 b and 350 d disposed at spaced apart positions along a lengthof the subject's thorax to the left and right of the subject's sternum,and additional pairings of electrodes (e.g., 351 a, 35 b) disposed at adifferent position along the length of the subject's thorax to the leftand right of the subject's sternum. As would be appreciated by thoseskilled in the art, the presence of multiple electrodes at differentspaced apart positions along the length and width of the subject'sthorax permits impedance measurements to be made in multiple axes.

FIG. 4 is a plan view of the probe 240 inserted into a lumen 14, 16 of asubject. The probe 240 includes at least one first electrode, and in theembodiment depicted, the probe 240 includes a pair of electrodes 341 a,341 b disposed at opposing positions along the length of the probe. Asnoted above, the probe 240 may include only a single electrode (e.g.,341 a), or alternatively may include multiple electrodes at variouspositions along the length of the probe. As illustrated in FIG. 4,conductive leads 441 a, 441 b are electrically connected to eachelectrode 341 a, 341 b, and may be routed from each electrode to theinterior of the probe 240 for ease of insertion and removal of the probe240, and for the comfort of the subject.

In use, the probe 240 is inserted into a lumen of the subject eitherprior to or during the performance of CPR. The probe may be insertedinto either the trachea of the subject, or into the esophagus of thesubject, each of which is centrally located with the body of thesubject, such that the arterial and venous portions of the circulatorysystem of the subject are disposed predominantly to the left and rightof the probe. Preferably the probe is made from a medical grade plasticor other suitable material that sufficiently rigid to permit insertioninto a lumen of the subject, but which can flex during application ofthe compressive forces of CPR. The plurality of additional electrodes350, 351, 352, which may be self-adhesive wet electrodes, may beattached to the skin of the subject's thorax in a well-known manner.

The at least one first electrode 341, 342, 343 of the probe 240 and theplurality of additional electrodes 350, 351, 352 are electricallycoupled to the electrode interface 230 (FIG. 2), and the impedancebetween different pairings of the electrodes is measured. Variousimpedance measurements may be made using the at least one firstelectrode 341, 342, 343 as a source electrode and using the plurality ofadditional electrodes 350, 351, 352 as receiving electrodes, or viceversa. For a particular impedance measurement, the at least one firstelectrode 341, 342, 343 of the probe may be used as a source electrode,and in another impedance measurement, the at least one first electrodemay be used as a receiving electrode. For example, and with reference toFIG. 3, electrode 341 a may be paired with electrode 350 a to determinethe impedance of the arterial vessels, and electrode 341 a may be pairedwith electrode 350 b to determine the impedance of the venous vessels.Where multiple additional electrodes are disposed at varying positionsalong the width and length of the subject's thorax, full impedancetomography may be performed and used to determine the directionality ofblood flow and changes in the directionality of blood flow over time.

During the performance of CPR, the impedance between various pairings ofelectrodes is monitored and recorded and used to determine variousmetrics relating to the ratio of arterial and venous blood in thecirculatory system of the subject. Such metrics may include the ratio ofthe arterial blood volume relative to the venous blood volume at aparticular instant of time as well as changes in the ratio of arterialblood volume relative to the venous blood volume over a period of time,for example, over successive 10 second intervals of time, overover-lapping sliding windows of time, or over the entire duration ofCPR. Other metrics that may be determined can include the rate offilling of the arterial and venous vessels over a short period of time(e.g. 0.1-2 seconds), changes in the rate of filling of the arterial andvenous vessels over successive periods of time period of time, a ratioof the capacity of arterial blood volume relative to the capacity ofvenous blood volume at a particular instant of time, changes in thecapacity of arterial blood volume relative to the capacity of venousblood volume over a period of time, the directionality of blood flow,and changes in the directionality of blood flow over a period of time.Such metrics can be indicative of the quality of CPR being performed ona subject, and further, such metrics may be used to modify the CPR beingperformed on the subject to enhance the quality of the delivered CPR.

For example, if the metrics indicate that the ratio of the arterialblood volume relative to the venous blood volume is low, the depth ofthe CPR compressions may be increased, or the time between compressionsmay be increased. The device may also recommend a shift in the optimallocation of the compression zone of the hands (where the CPR is beingperformed manually) or electromechanical compression device. Forinstance, it may be recommended to shift the location of the hands orcompression device to the subject's left hand side, just lateral to thesternum, where the hands or compression device will be placed moredirectly over the heart and arterial great vessels. Where the controlcircuit of the medical device is capable of communicating with anotherdevice, such as an electromechanical chest compression device,instructions may be sent to the other device to modify the depth or timebetween CPR compressions. Alternatively, where the ratio of the arterialblood volume relative to the venous blood volume is low, the controlcircuit of the medical device may send instructions to the other deviceto modify the location at which compressions are applied to the subject,or to modify the direction at which compressions are applied to thesubject to direct more blood to the arterial vessels. Where the metricsindicate that the arterial and venous blood vessels fill relativelyquickly with blood during the release phase of CPR compressions, thecontrol circuit may instruct the other device to increase the rate(i.e., frequency) at which CPR compressions are applied, and where themetrics indicate that the arterial and venous blood vessels fillrelatively slowly with blood during the release phase of CPRcompressions, the control circuit may instruct the other device todecrease the rate at which CPR compressions are applied.

Where CPR compressions are being performed manually, with the aid of aCPR coaching and feedback device, or with an electromechanical chestcompression device, the control circuit of the medical device mayvisually, audibly, or both visually and audibly present instructions formodifying the CPR being performed on the subject. For example, thecontrol circuit of the medical device may present instructions to modifythe depth and/or rate of CPR compressions, to shift the location atwhich compressions are applied to the subject, etc. Such instructionsmay even include a recommendation for the administration of apharmacological agent, such as a metabolite or metabolic enhancementagent, such as epinephrine. Alternatively, the control circuit of themedical device may communicate blood volume and flow data, or thevarious metrics to another device for storage or display, or to enablethe other device to monitor the quality of CPR being performed, or toprovide feedback or guidance to a rescuer. For example, where the otherdevice is an ALS defibrillator, the ALS defibrillator may store the dataand metrics to provide feedback to the rescuer during the performance ofCPR, or to permit a rescuer to later review information to betterunderstand how they performed with respect to the quality of CPRprovided.

In an embodiment, the control circuit is configured to issue arecommendation to modify a subject's intravascular fluid volume based atleast in part on at least one of the determination of the ratio ofarterial blood volume relative to venous blood volume and thedetermination of the rate of change of the ratio of the arterial bloodvolume relative to the venous blood volume over the period of time. Themodification may provide for an increase or a reduction in theintravascular fluid volume. In an embodiment, a user such as a nurse ora physician receiving the recommendation may provide fluid to thesubject, for example, intravenously. The provision of fluidintravenously may be provided through a venous access point, forexample. The fluid provided may be any fluid such as but not limited tosaline, crystalloid plasma volume expanders (ringer lactate, glucose),colloidal plasma volume expanders (albumin, hydroxyethyl starch,gelatin-based fluid), oxygen enriched blood, artificial blood products,therapeutic agents and/or the like.

FIG. 5 is a tomographic map of the electrical impedance of varioustissues of a human body. The conductances of the various tissues shownin FIG. 5 are approximately as follows:

Conductivity Tissue Type (ohms-cm) Skin 3.4 Blood 6.5 Lung 0.7 SkeletalMuscle 1.5 (transverse) 4.2 (longitudinal) Fat 0.5 Cardiac Muscle 7.6Bone  0.06

Conductivities of the various tissues can vary by as much as a factor of100. As can be seen in the isoadmittance curves shown in FIG. 6, theconductances of the internal organs, muscle and bone vary significantly,much more so than do conductances at the body surface. In accordancewith one embodiment of the present invention, electrical impedancetomography (EIT) is used to determine these internal conductances orimpedances. In this embodiment, electrical impedance tomography (EIT) isused to determine the resistivity distribution of the thorax in at leasttwo dimensions, and the calculated resistivity distribution is then usedto determine more accurately the relative volumes of the venous andarterial great vessels, e.g. the vena cava and aorta.

In the most basic implementation, only three electrodes with threepossible electrode pairs is sufficient to use EIT methods to determinemore accurately the relative volumes of the venous and arterial greatvessels. At least one electrode is placed in the thorax of the subject(tracheal or esophageal probe) with two electrodes placed externally onthe thorax at about the level of the sternal notch, approximately threeinches to the left and right of the sternal midline. In oneimplementation, seven electrodes are used (three internal and fourexternal, two on each side of the sternum, for a total of 5040 (7!)possible electrode pairs. This number is chosen for ease ofimplementation and cost; and other implementations with more electrodepairs are possible. For instance, three rows of electrodes may beapplied to the thorax so that a full three dimensional tomographicimpedance map can be calculated. This may become important as therelative location of the great vessels, in particular the aorta, shiftwith respect to the esophagus and trachea as they descend in thethoracic space toward the diaphragm.

The EIT system is governed by Poisson's equation:Δ·ρ⁻¹ ΔV=I,

Where V is the voltage, ρis the resistivity distribution and I is theimpressed current source distributions within the region being studiedand the boundary conditions are V₀ and J₀. In the case of EIT, highfrequency, low amplitude signals, e.g., 60 KHz and ˜1 microampererespectively, are used. Since there are no current sources of thisfrequency in the body, then ρ=0, and Poisson's equation becomesLaplace's equation:Δ·ρ⁻¹ΔV=0

In the field of EIT, several types of problems are studied:

1. The “forward problem”, where ρ, V₀ and J₀ are given and the goal isto determine the voltage and current distributions V and J.

2. The “inverse problem”, where V and J are given and the goal is todetermine p.

3. The “boundary value” problem where V₀ and J₀ are given and the goalis to determine ρ, V and J.

In one implementation, ρ, V and J are determined using boundary valueproblem methods, then once ρ is determined, the relative or absolutevolumes of the venous and arterial great vessels is determined

In general principle, the process of EIT involves injecting a current byan electrode, and the induced voltage is measured at multiple points onthe body surface.

In accordance with one implementation, what is termed the“multireference method” is used for configuring the current voltagepairs. (Hua P, Webster J G, Tompkins W J 1987 Effect of the MeasurementMethod on Noise Handling and Image Quality of EIT Imaging, Proc. Annu.Int. Conf. IEEE Engineering in Medicine and Biology Society 91429-1430.) In the multireference method, one electrode is used as thereference electrode while the remaining electrodes are current sourceswith the induced voltages being measured on each electrodesimultaneously while the current is being delivered. The amplitude ofthe current sources are individually varied and each electrode istreated as a reference lead in succession. Finite element methods arethen used to convert the calculus problem (Δ·ρ⁻¹ΔV=0) into a linearalgebra problem of the form YV=C, where Y, V, and C are the conductance,voltage, and current matrices respectively. Y, V, and C are alsosometimes known as the master matrix, node voltage vector, and nodecurrent vector respectively. Mesh generation is performed on the two orthree-dimensional physical model with triangular or quadrilateralelements for two dimensional problems and hexahedral shapes forthree-dimensional problems. Boundary conditions are then set such as atthe reference node or driving electrodes for Dirichlet (known surfacevoltages) or Neuman (known surface currents) boundary conditions. Anumber of methods have been used to compute the master matrix such asGaussian elimination or Cholesky factorization.

The Newton-Raphson algorithm may also be used for reconstruction of theresistivity distribution. The algorithm is an iterative algorithmparticularly well suited to non-linear problems. The Newton-Raphsonmethod minimizes an error termed the “objective function”. Here, it isdefined as the equally weighted mean square difference between themeasured and estimated voltage responses:Φ(ρ)=(½)(V _(e)(ρ)−V ₀)^(T)(V _(e) (ρ)−V ₀).

Using methods known to those skilled in the art, an algorithm isutilized whereby a distribution is first estimated, then the theoreticalvoltage response to a given current input is calculated using the finiteelement method. The estimated voltages are subtracted from the measuredvoltages to obtain the objective function. If the objective function isless than an error threshold, the estimated distribution is deemed to bean acceptable estimation. If not, the following equation is used toupdate the resistivity distribution:Δρ ^(k) =−[V _(e)′(ρ^(k))^(T) V _(e)′(ρ^(k))]⁻¹ {V _(e)′(p ^(k))^(T) [V_(e)′(ρ^(k))−V ₀]}

This sequence is repeated until an acceptable estimation is achieved.

In accordance with one implementation, a table lookup method is providedto determine the estimated voltage matrix V_(e)(ρ). The table values arebased on average patient resistivity distributions and assuming correctplacement of the electrode. Better accuracy can be achieved by providinganatomical markings on the electrode pads, such as using the sternalnotch to provide accurate lateral and vertical positioning relative tothe diaphragm.

Accuracy may also be improved by providing a secondary imaging methodsuch as ultrasound to take advantage of its higher imaging resolution tocalculate the positions of the internal organs relative to theelectrodes. If a secondary imaging method such as ultrasound is used todetermine the positions of internal tissues, EIT can be used todetermine the resistivities of each tissue type.

In other implementations, an average resistivity value is determined forthe tissue regions as defined by the secondary imaging method. This isaccomplished by first defining a tissue region such as the lungs ormyocardium by standard image processing methods. Next, the calculatedresistivity distribution is overlayed onto the secondary image. Allnodes of the resistivity distribution that are contained within aparticular tissue region are combined together into a single resistivitymeasure for that tissue region. The method of combination may be anaveraging, median, or other statistical or image processing method.

To further improve tomographic mapping accuracy additional electrodesmay be placed on the thorax, in particular towards the patient'sposterior.

In another implementation, a physiological parameter, e.g., theelectrocardiograph (ECG), is measured in conjunction with the EIT imageto determine if the electrical activity of the heart is causing bloodflow in the vessels, particularly on the arterial side. In this wayreturn of spontaneous circulation can be distinguished from pulselesselectrical activity during CPR.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the scope of theinvention. Accordingly, the foregoing description and drawings are byway of example only.

The invention claimed is:
 1. A medical device system comprising: a probeconfigured to be orally inserted into a lumen of a subject andconfigured to extend into a thorax of the subject, the probe includingat least one internal electrode: a plurality of external electrodesincluding: a first external electrode configured to be disposedexternally on the thorax of the subject on a first side of the sternumof the subject, and a second external electrode configured to bedisposed externally on the thorax of the subject on a second side of thesternum of the subject that opposes the first side; a control circuit,electrically coupled to the at least one internal electrode, the firstexternal electrode, and the second external electrode; and a feedbackdevice operatively coupled to the control circuit, wherein the controlcircuit is configured to: measure impedance values between the at leastone internal electrode and each of the first external electrode and thesecond external electrode; generate a tomographic map of the measuredimpedance values; and provide a feedback signal via the feedback deviceto provide blood volume information to a user based on the tomographicmap.
 2. The medical device system of claim 1, wherein the blood volumeinformation is a ratio of arterial blood volume relative to venous bloodvolume in a circulatory system of the subject.
 3. The medical devicesystem of claim 2, wherein the ratio of arterial blood volume relativeto venous blood volume is indicative of the quality of cardiopulmonaryresuscitation being performed on the subject.
 4. The medical devicesystem of claim 2, wherein the control circuit is further configured togenerate at least one metric relating to the ratio of arterial bloodvolume relative to venous blood volume in the circulatory system of thesubject.
 5. The medical device system of claim 4, wherein the at leastone metric relating to the ratio of arterial and venous blood in thecirculatory system of the subject comprises at least one of a ratio ofarterial blood volume relative to venous blood volume at a particularinstant in time, the ratio of arterial blood volume relative to thevenous blood volume over a period of time, a rate of filling of arterialand venous vessels over a short period of time, changes in the rate offilling of the arterial and venous vessels over successive periods oftime, a ratio of a capacity of arterial blood volume relative to acapacity of venous blood volume at a particular instant of time, changesin the capacity of arterial blood volume relative to the capacity ofvenous blood volume over a period of time, a directionality of bloodflow, and changes in the directionality of blood flow over a period oftime.
 6. The medical device system of claim 1, wherein the blood volumeinformation is a relative volume of blood of an venous great vessel anda relative volume of blood of an arterial great vessel of the subject.7. The medical device system of claim 6, wherein the relative volume ofblood of the arterial great vessel being greater than the relativevolume of blood of the venous great vessel is indicative of effectivecardiopulmonary resuscitation being performed on the subject.
 8. Themedical device system of claim 1, wherein the feedback device visuallydisplays the blood volume information on a display device.
 9. Themedical device system of claim 1, wherein the control circuit is furtherconfigured to: estimate a metric relating to a ratio of arterial andvenous blood in a circulatory system of the subject based upon thetomographic map of the measured impedance values; and provide acardiopulmonary resuscitation feedback signal via the feedback device tomodify cardiopulmonary resuscitation being performed on the subjectbased on the metric.
 10. The medical device system of claim 9, whereinthe cardiopulmonary resuscitation feedback signal comprises arecommendation that includes at least one of varying a rate ofcardiopulmonary resuscitation compressions, varying a depth of thecardiopulmonary resuscitation compressions, varying a position at whichthe cardiopulmonary resuscitation compressions are applied to thesubject, varying a direction at which the cardiopulmonary resuscitationcompressions are applied to the subject, changing a duration of timeover which each compression is applied to the subject, changing aduration of time over which compressive forces are released, andrecommending administration of a pharmacological compound.
 11. Themedical device system of claim 9, wherein the metric relating to a ratioof arterial and venous blood in a circulatory system of the subject isestimated from the tomographic map by determining a resistivitydistribution of the thorax of the patient in at least two dimensionsusing the tomographic map and estimating the metric relating to a ratioof arterial and venous blood in a circulatory system of the subject fromthe resistivity distribution.
 12. The medical device system of claim 9,wherein further comprising a communication interface by which themedical device system can communicate with an electromechanical chestcompression device, and wherein the control circuit is furtherconfigured to communicate the cardiopulmonary resuscitation feedbacksignal to the electromechanical chest compression device.
 13. Themedical device system of claim 12, wherein the cardiopulmonaryresuscitation feedback signal comprises instructions to at least one of:vary a rate of cardiopulmonary resuscitation compressions, vary a depthof the cardiopulmonary resuscitation compressions, vary a position atwhich the cardiopulmonary resuscitation compressions are applied to thesubject, vary a direction at which the cardiopulmonary resuscitationcompressions are applied to the subject, change a duration of time overwhich each compression is applied to the subject, and change a durationof time over which compressive forces are released.
 14. The medicaldevice system of claim 9, wherein the control circuit is furtherconfigured to issue a recommendation to modify an intravascular fluidvolume of the subject based at least in part on the determined metricrelating to the ratio of arterial and venous blood in the circulatorysystem of the subject.
 15. The medical device system of claim 1, furthercomprising a communication interface by which the medical device systemcan communicate with a resuscitation device, the resuscitation deviceincluding at least one of a chest compression device, a CPR coaching andfeedback device, and a defibrillator.